Analog to digital signal converters (ADCs) are known. Such converters are normally necessary where a digital device, such as a digital computer, must process inputs from non-digital signal sources. Examples of the use of such converter devices include the processing of analog inputs to digital communication systems, automobile engine sensors, and analysis of seismic information from seismic sensors.
ADCs typically provide a digital output proportional to the analog input. The number of bits available within the binary word, output from the device, typically provide a measure of resolution of the measured signal. Such resolution within the ADC is provided by a series of interrelated voltage references and/or voltage dividers within the ADC. Such references within the ADC are compared to an input during ADC operation under an algorithm providing a digital output proportional to the input. During manufacture the relative values of the reference voltages may be produced by a manufacturing process using etching, laser trimming, or other appropriate manufacturing technique.
To insure overall performance characteristics, ADCs are typically subject to the initial calibration during manufacture to ensure accuracy at two voltage extremes (high and low). Calibration at the two extremes insures that the output of the A/D is relatively accurate at the two extremes. Input voltage values between the two extremes, on the other hand, may exhibit considerable linearity error. The relative value of the linearity error between the two extremes is related to the manufacturing used to produce the proportional reference voltages.
Where reference voltages are produced using an etching process, linearity may be of a lesser quality. Where laser trimming is combined with etching then quality may be significantly improved and may offer a high-performance alternative where linearity requirements are restrictive in nature. Such high-performance alternatives, on the other hand, can be prohibitively expensive.
Total device accuracy is, typically, specified within the product literature of a device. Inaccuracies relative to non-linearity of device response may also be specified within device literature under a variety of different measuring methodologies. Accuracy may also drift, with time, as the ADC ages.
While ADCs perform well in converting analog data to a digital format, the accuracy of such devices may vary depending upon the magnitude of the sensed signal, and with time. ADCs may also vary in accuracy, from device-to-device. In addition, individual ADCs are sometimes constructed with phased clocks to increase their effective sampling rate, or in a composite offset structure, to increase their effective number of bits of resolution. Since multiple ADCs never have the same linearity characteristics, the composite structures suffer even worse linearity than any of their individual components. This creates particularly poor performance in spectral applications. Because of the importance of ADCs in signal processing a need exists for a method of improving the accuracy of A/D output data without resort to high-performance alternatives.